Numerous processes have become known for the preparation of polyurethane dispersions in water. Summarizing reports have been given, for example, by D. Dieterich and H. Reiff in "Die Angewandte Makromolekulare Chemie" 26, 1972 (pages 85-106); D. Dieterich et al in Angewandte Chemie, 82, 1970 (pages 53-63) and D. Dieterich et al in J. Oil Col. Chem. Assoc. 53, 1970 (pages 363-379). These reports also give a comprehensive survey of the literature. The process which is generally preferred for the preparation of aqueous polyurethane dispersions comprises reacting an isocyanate prepolymer dissolved in an organic solvent with a chain lengthening agent. Either the prepolymer used for this process or the chain lengthening agent contains ionic groups or groups which are capable of forming ions and which are subsequently converted into ionic groups or which are converted into ionic groups in the course of the polyaddition reaction.
The aqueous dispersion is formed at the same time or subsequently by the addition of water and removal of the organic solvent by distillation. It is apparent that the solvent used in this procedure must have a lower boiling point than water.
Polyurethane resins having a specific structure cannot generally be synthesized in organic solvents or dispersing agents which contain hydroxyl groups because the polyaddition reaction which takes place under those conditions is a non-specific one, i.e., the hydroxyl-containing solvent or dispersing agent is liable to take part in the reaction as well.
Particular diisocyanate polyaddition products directly prepared in polyethers or polyesters as dispersing agents are, however, known. Thus, according to German Auslegeschrift 1,168,075 U.S. Pat. No. 3,360,495, isocyanates are reacted with difunctional primary alcohols in a polyether or polyester (molecular weight 500 to 3,000) as the dispersing medium, the polyether or polyester used being one which contains at least two secondary hydroxyl groups in the molecule. According to German Auslegeschrift 1,260,142 U.S. Pat. No. 3,325,421, compounds containing isocyanate and amino groups are made to undergo a polyaddition reaction in situ in a polypropylene glycol ether used as the dispersing agent. However, since, as already explained above, a certain amount of dispersing agent (depending on the reaction conditions) also takes part in the reaction, the products obtained in the processes according to German Auslegeschriften Nos. 1,168,075 and 1,260,142 are highly viscous dispersions which may be used as thickeners for textiles or dyeing processes. Preparation of the dispersions is, moreover, restricted to those products in which the dispersing agent and the chain lengthening agent used for the polyaddition reaction differ markedly from each other in their reactivity with isocyanates.
A 20% polyhydrazodicarbonamide dispersion in a polypropylene glycol ether prepared according to German Auslegeschrift No. 1,260,142 already has a viscosity of over 200,000 cP at 25.degree. C. This amounts to more than 200 times the viscosity of pure dispersing agent. When attempts are made to prepare a 40% dispersion, the reaction mixture generally solidifies before the polyaddition has been completed. The high viscosities which occur already at relatively low solid concentrations severely restrict the possibilities of using the products of the process since in many fields of application they cannot be dosed with the aid of the usual dosing apparatus. For producing polyurethane foams, for example, for which purpose such dispersions could be suitable, the viscosities of the starting materials must be below 2500 cP if conventional high pressure machines are to be employed.
According to another earlier proposal, relatively low viscosity dispersions of polyureas and/or polyhydrazodicarbonamides in polyethers having hydroxyl groups can be obtained if the polyaddition reaction is carried out continuously in continuous flow mixers. The process has the disadvantage of requiring a relatively complicated technique of dosing and mixing which is uneconomical for usual production rates. Moreover, in some cases high solid concentrations may give rise to considerable difficulties in the removal of the heat of reaction. It is clear that only special types of dispersion can be produced by this process, for the reason explained above. Polyisocyanate polyaddition products having a defined structure also cannot be produced by this process since the possibility of the hydroxyl containing dispersing agents taking part in the reaction cannot be excluded.